In all the Iron Man movies, there are shots of Tony talking to J.A.R.V.I.S. while wearing the armour, and he moves his head to look at different things that pop up.
The suit is roughly the size of a human body, just slightly bigger so a man can fit inside. Is there any canonical reason as to why he has room to move his head inside the helmet when the helmet is about the size of his head?
He doesn't, it's an optical illusion.
The camera which is showing inside the suit remains at a flat angle, and when Tony moves his head, the helmet moves with it, but the camera is not attached to his head nor his helmet.
In order to look at a different part of the screen, Tony just moves his eyes, and you can see that when he moves his head, the HUD moves with him which indicates that he isn't moving his head inside the helmet:
Iron Man's HUD movement as seen in The Avengers (2012)
The depictions of the helmet interiors vs exteriors don't match up. Even if the helmet is moving with his head in the interior shots, it nevertheless depicts a certain amount of clear space inside the helmet. Where he dons or removes the helmet, it appears that no such space exists.
From an in-universe perspective, it could be some form of virtualized holographic projection (into space outside the helmet).
From an out-of-universe perspective, we need to see Tony's face to see him hard at work, anxious, etc. to keep our connection to his character. To get those shots, some creative license is required, so we have to suspend our disbelief.
